When imaging the surrounding image information over a wide range using a small number of cameras, in general, a wide angle optical system having a large imaging angle of field (wide angle of field) is used. For example, such wide angle optical systems are used in monitoring or automobile applications.
Further, for example, in the case of a rear view monitor for automobiles, etc., in addition to being wide angle, it is required that, from the point of view of practical applicability, it does not block the field of view of the driver, and is compact from the point of view of beauty of the automobile.
Such wide angle optical systems have been disclosed, for example, in Patent Document 1. In the wide angle optical system disclosed in this Patent Document 1, shading that becomes a problem in the case of ultra wide angle has been prevented by using a small number of lenses such as 4 to 5 lens elements.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2005-227426.